I heard someone talking about the non-viability of electric airplanes. One of the points they referenced was that the longer a conventional plane flies, the less it weighs due to fact that it’s burning fuel and that a battery doesn’t weigh less as it discharges.
I don’t recall how that was relevant to electric planes but it did make me curious, do batteries really not weigh less as they discharge?
I’m sure if they do, it’s so miniscule as to be negligible and so maybe the speaker was talking in practicalities, but still.
Batteries are inherently heavy. More heavy than the weight of equivalent amount of liquid fuel to generate similar power. I think generally for a typical passenger vehicle, the EV version is about 30% heavier.
Translate that into a jet aircraft that carries as much fuel weighing as much or more than the total plane weighs less the fuel. Ergo a battery powered jet would prohibitively weigh more than it would likely be able to take off.
The question of weight loss during discharge of batteries has been addressed. As a matter of physics it surely does happen. As a matter of engineering the change is negligible.
As applied to airplanes …
For any given size and shape of airplane, the heavier it is, the more power it takes to fly and the worse “gas mileage” (loosely speaking) it gets while doing so.
When you put a pound of payload on a passenger or cargo airplane, you’ll consume extra fuel to lug that pound of payload all the way to the destination.
For something like a bomber, where you’ll drop the payload along the way, you’ll pay the extra weight penalty only halfway. Then you’ll get better “gas mileage” coming home.
Fuel itself is a bit like the bomber but the weight reduction is continuous; not one big lump near the halfway point in the flight.
You load all the fuel at the beginning and at first you’re consuming lotsa extra fuel to carry all that fuel weight. As fuel is burned and the fuel weight gets lighter, the gas mileage improves incrementally and the incremental fuel consumption to carry the remaining fuel declines. By the time you get to the destination after a long flight, you may have used 90% of the fuel you started with and the “carrying charge” in incremental fuel consumption to carry that last 10% of your original fuel has declined similarly.
The net result over a long flight is that the “carrying charge” for the fuel weight declines over time and the overall end-to-end average is less than the instantaneous “carrying charge” right at takeoff time with full fuel weight. Often much less, like around half.
Setting aside all concerns about the inherent weight or energy density of current-tech batteries vs jet fuel, you can see that the in-flight reduction of “carrying charge” over time won’t occur under battery power. It’ll be zero. That is a significant difference that affects a lot of other things in aircraft design.
As was said upthread already, current-tech batteries are much heavier and much bulkier than is jet fuel for equivalent usable energy. That too is a big problem. But it’s a different problem from the carrying charge problem.
I took a drone flying course recently, and one of the drone manuals actually claimed the battery would weigh less as the flight progressed. Nobody in the class believed that for a second.
Myth no. 3: electricity has zero mass and zero weight
Though the terms “intuitive” and “common sense” are trendy now, we can’t always let ourselves be guided by our first impression. If you ask someone how much electricity weighs, they’ll probably tell you nothing.
Reality. The fact that we can’t see it does not mean that electricity does not have mass or weight. The movement of electrons has mass and weight, though they’re almost invisible.
Google doesn’t say that. What’s the actual source?
Back on topic, one proposal for getting better energy density out of batteries is “air-breathing” batteries, that would use a chemical reaction between the battery materials and atmospheric oxygen (the fact that hydrocarbon fuels can make use of atmospheric oxygen is a large part of why they’re so much more energy-dense than batteries). But if you want your air-breathing battery to be re-useable, you’d need to keep the oxides (whatever they are) in the battery, so such a device might actually get heavier as it’s discharged.
Similarly to this, solid state memory gets negligibly heavier as it’s filled with data. Someone did a calculation with a Kindle. Of course other factors contribute much more to the weight fluctuation of the device. There was a thread about that years ago.
Note that an electric plane doesn’t have to run off batteries; it can generate electricity on board (just like BEV vs FCEV cars.) There are supposedly some advantages to electrification that I don’t understand or recall but probably referenced in another thread.
I also once discussed giant extension cords with some folks at a federal research funding agency. There may have been alcohol involved.
But even if assuming batteries, there’s still some potential viability with smaller aircraft.
This is what I was thinking, maybe there would be electric catapults on the runway, and then a separate module that could glide to a suitable collection point for recharging. Maybe that happens upon reaching cruise altitude, or whatever point seems optimal to energy consumption & logistics.